[REM] python-stdnum from requirements

It is never used in the code, and causes problems with wheels: related
to pypa/pip#2105

closes #3744
[FIX] web: force company logo refresh per db

The company_logo is cached and was not refreshed when switching between database, still showing the logo of the company of the previous database
Fixes #2738
[FIX] decimal_precision: avoid refreshing float precisions at each cache invalidation

Changing the decimal precision of float fields is a rare
operation, while cache clearing occurs fairly frequently.
Signaling a full registry change when the decimal precision
is changed (instead of a mere cache change) is therefore
a better trade-off, and more semantically correct as well.
This way we avoid the decimal precision refresh for each
invalidation.

Registry invalidation implies cache invalidation.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64